The Long Term Evolution (LTE) system is currently under standardization by the 3GPP, and in the LTE system, Orthogonal Frequency Division Multiplexing (OFDM) is adopted at the physical layer, a dedicated channel is dispensed with, and physical resources are shared among all users and allocated by a Base Station (The term of “Base Station” should be understood as being synonymous with “evolved Node B” in the present invention).
According to the current LTE specification, a cell handover flow of a User Equipment (UE) in an LTE system as illustrated in FIG. 1 includes the following operations S101 to S105.
S101. The UE transmits to a source cell a measurement report including information on a measurement result, e.g., reception strength of a pilot signal, etc.
S102. The source cell transmits to a target cell a cell handover request including context information of the UE upon reception of the measurement report.
S103. The target cell generates a cell handover command from the cell handover request, and transmits to the source cell an acknowledgment message for the cell handover request, which includes the cell handover command.
S104. The source cell forwards the cell handover command to the UE.
S105. The UE performs a random access and a subsequent process in response to the cell handover command.
In the LTE system, it is not desirable for the UE to fetch system information of the target cell during a handover in order to reduce a delay due to an interruption of the handover, therefore, certain system information of the target cell has to be put into the cell handover command and transmitted from the source cell to the UE.
During a handover, the UE performs a random access to the target cell for the purpose of acquiring uplink synchronization information in the target cell and transmitting a handover completion message to the target cell. A specific random access may be performed in the following two modes a) and b).
a) In a contention-based mode, a specific random access flow as illustrated in FIG. 2 includes the following operations S201 to S204.
S201. A UE transmits a random access preamble (which is referred to as Msg1 hereinafter) to an evolved Node B (eNB) in the uplink direction, where the Msg1 is transmitted over a Physical Random Access Channel (PRACH).
S202. The eNB transmits a random access response (which is referred to as Msg2 hereinafter) in the downlink direction upon reception of the Msg1.
S203. The UE performs uplink scheduled transmission (which is referred to as Msg3 hereinafter) over an uplink resource indicated in its received random access response.
The foregoing Msg2 generally functions to acknowledge the Msg1, feed back timing adjustment information and allocate dynamically the uplink resource for the Msg3. The time when the Msg2 is transmitted is scheduled dynamically by the eNB. Msg2s for a plurality of UEs may be combined for transmission. The duration of a resource required by the Msg2 is one timeslot (TTI), i.e., a sub-frame.
The Msg3 includes upper layer signaling, e.g., a handover confirmation message (Handover Confirm).
S204. The eNB transmits a contention resolution message (which is referred to as Msg4 hereinafter) in the downlink direction upon reception of the Msg3. The Msg4 generally carries a unique identifier of the UE succeeding in the contention.
b) In a contention-free mode, a specific random access flow as illustrated in FIG. 3 includes the following operations S301 to S303.
S301. An eNB at a network side allocates a dedicated random access preamble for a UE.
S302. The UE transmits the dedicated random access preamble (which is referred to as Msg1 hereinafter) in the uplink direction.
S303. The eNB transmits a random access response (which is referred to as Msg2 hereinafter) in the downlink direction upon reception of the random access preamble.
Since the random access preamble is dedicated here, no contention resolution is required.
In the foregoing random accesses, the UE has to know a parameter associated with a control channel of the target cell in order to complete the random access for the following reasons.
In either of the contention-based random access or the contention-free random access, the UE has to receive a Msg2 (a random access response) transmitted over a Physical Downlink Shared Channel (PDSCH), the resource of which is allocated over a Physical Downlink Control Channel (PDCCH) in an LTE system, therefore, the UE has to know in advance a parameter associated with the PDCCH of the target cell during the random access.
For the contention-based random access, the UE has to transmit the Msg3 to which Hybrid Automatic Repeat reQuest (HARQ) transmission is applicable, thus the UE has to monitor whether a Hybrid ARQ ACKnowledgment/Non-ACKnowledgment (HARQ ACK/NAK) signal is fed back by the eNB over a Physcial Harq Information Channel (PHICH), therefore, the UE has to know in advance a parameter associated with the PHICH of the target cell during the monitoring.
For the contention-based random access, the UE has to receive the Msg4 to which HARQ transmission is applicable, thus the UE has to transmit an acknowledgement (ACK) signal corresponding to the Msg4 over a Physcial Uplink Control Channel (PUCCH), therefore, the UE has to know in advance a parameter associated with the PUCCH of the target cell during transmitting the ACK signal.
Timeslots of an LTE Time Division Duplex (TDD) system in the prior art are configured as illustrated in FIG. 4, where a radio frame with a length of 10 ms includes two half-frames each with a length of 5 ms, each half-frame includes four normal sub-frames each with a length of 1 ms and a special sub-frame with a length of 1 ms, and the special sub-frame includes three special timeslots (a DwPTS, a GP and an UpPTS).
A period of the timeslot may be configured as 5 ms and 10 ms respectively.
1) For the configured timeslot period of 5 ms, the two half-frames in a radio frame are identical. Sub-frames 0 and 5 are configured fixedly for the downlink, sub-frames 2 and 7 are configured fixedly for the uplink, and sub-frames 3 and 8, and subframes 4 and 9 may be configured for the uplink or downlink.
2) For the configured timeslot period of 10 ms, the two half-frames in a radio frame are different. Sub-frames 0 and 5 are configured fixedly for the downlink, sub-frames 2, 7, 8 and 9 are configured fixedly for the uplink, and sub-frames 3 and 4 may be configured for the uplink or downlink.
Lengths and usages of the DwPTS, the GP and the UpPTS may be configured respectively as required.
Accordingly, for an LTE TDD system, TDD timeslot configuration information includes the configured timeslot period (5 ms or 10 ms), the uplink or downlink direction of each normal sub-frame, and the configured length and usage of each special timeslot in the special sub-frame.
For a TDD system, a value of a parameter of one or more control channels (e.g. the PDCCH, the PHICH and the PUCCH) is associated with the TDD timeslot configuration.
For example, in the case of the PUCCH (over which an uplink HARQ feedback is carried), the structure of the PUCCH varies with the TDD timeslot configuration, and an improper interpretation (i.e. improper correspondence relationship) may arise due to ignorance of a proportion of timeslots. For example, there exist following two TDD timeslot configurations.
Referring to FIG. 5, the proportion of TDD uplink timeslots to TDD downlink timeslots, i.e., DL:UL is equal to 4:3, and here a feedback signal of the downlink sub-frame 5 is put in the uplink sub-frame 1.
Referring to FIG. 6, the proportion of TDD uplink timeslots to TDD downlink timeslots, i.e., DL:UL is equal to 5:2, and here a feedback signal of the downlink sub-frame 5 is put in the uplink sub-frame 2.
The UE cannot identify the specific uplink sub-frame in which a feedback signal of the downlink sub-frame 5 is put due to ignorance of the proportion of uplink timeslots to downlink timeslots. Consequently, the UE has to know the TDD timeslot configuration information in order to deduct a parameter of a control channel.
In the prior art, however, system information of a target cell needs not to be fetched by a UE, therefore, if a handover target cell operates in the TDD mode, then the UE cannot acquire TDD timeslot configuration information of the target cell, and thus cannot deduct some parameters of a control channel involved during the random access, so that neither the random access nor the handover can be completed, thus hindering normal operation of the system. Further, as currently known, the contents of system information of the target cell carried in a cell handover command during a random handover have not been specified.
In summary, a cell handover can not be implemented in the prior art in the case that the target cell operations in the TDD mode.